Bulk renaming armature bones after already creating them?
Open a text editor in Blender and paste the code below there.
Replace the text in "original" and "replacement" as needed.
Go to pose mode and select the parent of the new chain.
Press "Run Script" in the text editor.


Here's the code:

import bpy
original    = "FrontLeg"    # Original name that was duplicated to generate all legs
replacement = "2ndFrontLeg" # The new name you want for this particular leg's bone chain

parent = bpy.context.active_pose_bone
for b in [ parent ] + parent.children_recursive:
    b.name = replacement